A galvannealed steel sheet is excellent in weldability and paintability as compared to a galvanized steel sheet. Therefore, the galvannealed steel sheet is widely used in a wide range of fields of automobile vehicle bodies, and further home electric appliances, construction materials, and the like. The alloyed hot-dip galvanized steel sheet to be used for such usages is made available for use after being press formed normally.
In a manufacturing method of the alloyed hot-dip galvanized steel sheet, immediately after hot-dip galvanizing is performed on the surface of a steel sheet, heating to the melting point of zinc or higher and holding are performed to make Fe in the steel sheet diffuse into a plating layer. Then, alloying reaction with Zn is caused to generate a Zn—Fe alloy phase. However, such an alloyed hot-dip galvanized steel sheet has the disadvantage of being poor in press formability as compared to a cold-rolled steel sheet.
The cause of poor press formability lies in a structure of an alloyed hot-dip galvanizing layer. That is, a Zn—Fe alloy plating layer formed by making Fe in the steel sheet diffuse into the plating layer to cause alloying reaction with Zn is a plating layer composed of a Γ phase 11, a Γ1 phase 12, a δ1 phase 13, and a ζ phase 14 formed on a base iron 10 as schematically illustrated in FIG. 1 normally. Further, this plating layer changes in the order of the Γ phase→the Γ1 phase→the δ phase→the ζ phase as the Fe concentration becomes lower.
As for the hardness of these phases, it is about 505 Hv in the Γ1 phase in Vickers hardness, which is the highest, and next it is about 326 Hv in the Γ phase, it is about 284 to 300 Hv in the δ phase, and it is about 200 Hv in the ζ phase. Particularly, the Γ phase and the Γ1 phase existing in a plating layer region close to the surface of the steel sheet (at a plated steel sheet interface) are hard, and in an upper region of the plating layer, the soft ζ phase is generated.
The ζ phase is soft, likely to adhere to a press die, has a high friction coefficient, and is poor in slidability. Therefore, the ζ phase results in the trigger that causes a phenomenon in which the plating layer adheres to a die to peel off when severe press forming is performed, (which will be flaking, hereinafter). Further, the Γ phase and the Γ1 phase are hard and brittle, to thus result in the trigger that causes a phenomenon in which the plating layer turns powdery to peel off during press forming, (which will be powdering, hereinafter).
It is important that the slidability should be good when the galvannealed steel sheet is press formed. Therefore, in terms of the slidability, as the plating layer, a film having a high Fe concentration that is alloyed to a high alloying degree, has high hardness, has a high melting point, and is unlikely to cause adhesion is effective, but powdering becomes likely to be caused.
On the other hand, when a plating layer having a low Fe concentration that is alloyed to a low alloying degree and suppresses generations of the Γ phase and the Γ1 phase is employed in order to prevent the powdering, the slidability deteriorates and the flaking becomes likely to be caused.
In order to improve the press formability of the galvannealed steel sheet, the properties contrary to each other, which are slidability and powdering, are required to be both achieved.
As a technique to improve the press formability of the galvannealed steel sheet, there has been so far proposed a method of manufacturing a δ1-based galvannealed steel sheet by performing plating in a high-Al bath at a high impregnating sheet temperature defined by the relation to the concentration of Al to suppress alloying reaction, and then performing an alloying treatment so that an outlet side sheet temperature becomes higher than 495° C. to 520° C. in an alloying furnace in a high-frequency induction heating system (see, for example, Patent Literature 1). Further, there has been also proposed a manufacturing method of a galvannealed steel sheet in which hot-dip galvanizing is performed to be immediately held for 2 to 120 seconds in a temperature region of 460 to 530° C., and then is cooled to 250° C. or lower at a cooling rate of 5° C./second or more to form an alloyed plating layer with a δ1 single phase (see, for example, Patent Literature 2). Further, in order to achieve both surface slidability and powdering resistance, there has been also proposed a manufacturing method of a galvannealed steel sheet in which in an alloying treatment when manufacturing the galvannealed steel sheet, a temperature pattern of the alloying treatment is determined based on a temperature distribution obtained by multiplying each temperature (T) and each time (t) during heating⋅cooling together and adding the resultants (see, for example, Patent Literature 3).
The object of each of these prior techniques is to, by controlling the alloying degree, achieve hardening of an alloyed hot-dip galvanizing layer and attain achievement of both powdering resistance and flaking resistance to be disadvantages during press forming of the galvannealed steel sheet.
Further, the slidability is greatly affected by a surface flat portion, and therefore there has been proposed a technique of obtaining a galvannealed steel sheet excellent in slidability that has good powdering resistance even in a plating film with a lot of ζ phases existing in a surface layer by controlling a surface flat portion (see, for example, Patent Literature 4).
The object of this technique is to obtain a galvannealed steel sheet excellent in slidability that has good powdering resistance even in a plating film with a lot of ζ phases existing in a surface layer by decreasing the alloying degree. However, it is conceived that further improvement in slidability resistance is important because the flaking resistance (slidability resistance) is not sufficient.
Further, as a method of improving press formability of a zinc-based plated steel sheet additionally, there has been widely used a method of applying a high-viscosity lubrication oil. However, there are caused problems such that because the lubrication oil is high in viscosity, painting defects occur in a painting process due to degreasing failure and press performance becomes unstable due to a lack of oil during pressing.
Therefore, there have been proposed a technique of forming a ZnO-based oxide film on the surface of a zinc-based plated steel sheet (see, for example, Patent Literature 5) and a technique of forming an oxide film of Ni oxide (see, for example, Patent Literature 6). However, there is a problem that these oxide films are poor in conversion treatability.
Thus, there has been proposed a technique of forming an Mn-based oxide film as a film whose conversion treatability is improved (see, for example, Patent Literature 7). However, in each of these techniques of forming the oxide-based film, the relationship with the structure of an alloyed hot-dip galvanizing layer has not been examined concretely.
In Patent Literature 8, pre-plating has been proposed, but only the powdering resistance has been evaluated and no improvement of the flaking resistance has been made. Further, in Patent Literature 9, a Γ2 phase has been proposed, but only the powdering resistance has been evaluated and no improvement of the flaking resistance has been made. Further, in Patent Literature 10, evaluations of the powdering resistance and the slidability have been performed, but further stability is sometimes required at the time of press forming such that a sheet thickness is reduced practically.